As typically seen in a liquid crystal display device, a thin film transistor (TFT) formed over a flat plate such as a glass substrate is generally manufactured using a semiconductor material such as amorphous silicon or polycrystalline silicon. TFTs manufactured using amorphous silicon have low field-effect mobility, but can be formed over a large glass substrate. On the other hand, TFTs manufactured using crystalline silicon have high field-effect mobility, but due to the necessity of a crystallization step such as laser annealing, the transistors are not always suitable for being formed over a large glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a TFT is manufactured using an oxide semiconductor as a semiconductor material and applied to an electronic appliance or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a TFT is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used as a switching element or the like of an image display device.
The field-effect mobility of a TFT in which a channel formation region is formed in an oxide semiconductor is higher than that of a TFT using amorphous silicon. An oxide semiconductor film can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and the manufacturing process of the TFT including an oxide semiconductor is simpler than that of the TFT using polycrystalline silicon.
TFTs which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and an electronic paper.
In the case where the TFTs including an oxide semiconductor are applied to a display device, the TFTs can be applied to, for example, TFTs included in a pixel portion or TFTs included in a driver circuit. A driver circuit of a display device includes, for example, a shift register circuit or a buffer circuit, and the shift register circuit and the buffer circuit include a logic circuit. Therefore, by using a TFT including an oxide semiconductor as a TFT in the logic circuit, the driving speed of the driver circuit can be improved.
In the above display device, there is a problem in that unwanted charge build-up is caused in elements, electrodes, or wirings during manufacture or operation. In the case of a transistor, for example, such charge build-up will generate a parasitic channel which allows leakage current to flow. Further, in the case of a bottom gate transistor, charge may build up on a surface of or in a back channel portion in a semiconductor layer (i.e., a region of a semiconductor layer which is sandwiched between a source electrode and a drain electrode which are formed over the semiconductor layer) and generate a parasitic channel, in some cases. In addition, an oxide semiconductor has a relatively wide band gap as a semiconductor; accordingly, when an oxide semiconductor is used for a channel formation layer of a transistor, the transistor has high off resistance. Consequently, in the transistor whose channel formation layer includes an oxide semiconductor, unwanted charge build-up is likely to occur, and thus a parasitic channel is likely to be generated and leakage current is likely to flow. Accordingly, in order to realize desired operation of a driver circuit and a pixel portion, unwanted charge build-up, which is a cause of a parasitic channel, is preferably small.
[Reference]
    [Patent Document 1] Japanese Published Patent Application No. 2007-123861    [Patent Document 2] Japanese Published Patent Application No. 2007-96055